Events sharing an arc name form a thread: they get a coloured arc badge, a position in the thread (e.g. 2 of 5), and can be viewed on their own with the arc filter.
Start and end are both optional — type them as 5:35 or 1:02:03, or leave them in the link. For YouTube, an end time makes the player stop at that moment (a clip window). Twitch embeds support a start only.